Conventional earth formation porosity measurements are typically performed using chemical sources where there is a constant emission of neutrons with a specific energy spectrum. In such applications, formation porosity may be determined with a single set of neutron detectors that are sensitive to neutrons of only one type of energy—thermal or epithermal. Epithermal neutrons are typically detected using neutron detectors that are encapsulated inside high thermal capture cross section material. With that, the low energy tail of neutrons (i.e., predominantly the thermal neutrons) will be absorbed before they can reach active volume of the detectors. Accordingly, predominantly epithermal neutrons will be detected by these encapsulated detectors. Unfortunately, detection of epithermal and thermal neutrons for porosity measurements now requires two sets of detectors, one set shielded and one set unshielded, with the accompanying consequence of requiring premium space for the additional set of detectors in a space-limited downhole tool.